HTML emails I create in Marketo don't always come out as I intended on Outlook 2010, especially the Mac version. It takes some tweaking. I test multiple times. Also, not all CSS styles work on HTML emails, i.e. CSS positioning. I try to design my HTML in the simplest way possible. I've actually forgone CSS positioning and use tables instead. There are many resources online that highlight which CSS is ignored by HTML emails.
My suggestion is to code the email in basic HTML.